The short version
The short version of this review
Medik8 C-Tetra is a lightweight, water-free vitamin C oil with 7% tetrahexyldecyl ascorbate and vitamin E, directed for morning use before sunscreen. It cost £45 for 30ml, or £15 for an 8ml travel size, on Medik8 UK on 25 September 2026. The formula contains grapefruit peel oil.
Product information reviewed 2026-09-25.
01 / The basics
What is C-Tetra?
Medik8 lists 7% tetrahexyldecyl ascorbate, an oil-soluble vitamin C derivative, with vitamin E in a lightweight oil; the brand directs four drops each morning after cleansing, then sunscreen, and a patch test 24 hours before first use. Grapefruit peel oil is in the formula.
- Best for
- A water-free morning vitamin C step
02 / Review
What is in C-Tetra?
C-Tetra uses tetrahexyldecyl ascorbate, an oil-soluble derivative of vitamin C, at 7% in a lightweight, water-free oil with vitamin E. Derivatives like this are more stable than L-ascorbic acid in the bottle, but there is less published evidence on how much becomes active vitamin C in skin.
Medik8 sells stronger siblings — C-Tetra Luxe at 14% and C-Tetra Advanced at 20% — built on the same derivative.
03 / Review
Who is C-Tetra best for?
Readers who prefer an oil texture in the morning, or who have found low-pH L-ascorbic acid serums uncomfortable, and anyone wanting a mid-priced first vitamin C with a travel size to try first.
04 / Review
What should you check before buying?
Despite the “no artificial fragrance” wording, grapefruit peel oil adds fragrance allergens — relevant for fragrance-sensitive skin. The 7% derivative is not directly comparable with 15% or 23% L-ascorbic acid. Let the oil settle before sunscreen so the layers do not slide.
05 / How to use it
How should you use C-Tetra?
- 01Patch test 24 hours before first use
- 02Apply four drops in the morning after cleansing
- 03Allow to settle, then apply sunscreen
06 / Quick facts
What should you know before choosing C-Tetra?
- —7% tetrahexyldecyl ascorbate, an oil-soluble vitamin C derivative, with vitamin E
- —Lightweight, water-free oil in a glass bottle with pipette
- —£45 for 30ml, or £15 for an 8ml travel size, on Medik8 UK (25 September 2026)
- —Morning use: four drops after cleansing, then sunscreen
- —“No artificial fragrance”, but grapefruit peel oil and the fragrance allergens limonene, citral, linalool and geraniol are listed
- —Patch test 24 hours before first use, per the brand